Various instruments have found use for applications requiring level sensing, particularly sensing of discrete levels of, for example, a liquid or bulk material in a vessel. Such instruments include float level switches, ultrasonic level switches and capacitance point level switches. With any such instrument it is desirable that the instrument be adaptable to a plurality of different types of materials, and provide reliable, repeatable measurements.
With a capacitance-type level sensor, the instrument includes a circuit connected to a probe adapted to be disposed in a material containing vessel. Such a material acts as a dielectric. Assuming that the vessel is conductive, a capacitance is present between the probe and the vessel, or an alternative reference, which such capacitance represents level in the vessel.
A capacitance point level switch is operable to sense a specific level of the material in the vessel and provide a discrete output, such as with a relay contact, when the material is at a sufficient level to cause the capacitance between the probe and the reference to exceed a reference value. It is desirable that the circuit have the ability to detect minute changes in capacitance to provide a precise measurement.
The present invention is intended to satisfy the above requirements.